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We report a microelectromechanical systems contour-mode film bulk acoustic resonator (C-FBAR) to monitor in-vitro whole blood coagulation in real time. The C-FBAR has a suspended ring made of piezoelectric aluminum nitride excited in the radial-extensional mode. It operates at its resonant frequency of 150 MHz and possesses a quality factor of 77 in citrated human blood. The C-FBAR is characterized using aqueous glycerine solutions showing that it accurately measures the viscosity in the range of 1 to 10 centipoise. The C-FBAR, then, is used to monitor in-vitro blood coagulation processes in real time. Results show that its resonant frequency decreases as viscosity of the blood increases, during the fibrin generation process after the coagulation cascade. The coagulation time and the start/end of the fibrin generation are quantitatively determined. The C-FBAR has the potential to become a low-cost, portable, yet reliable tool for hemostasis diagnostics.